Immune suppression of self-reactive CD4+ and CD8+ T cells by natural, thymic-derived CD4+CD25+ T regulatory cells is crucial for the maintenance of peripheral self-tolerance. Recent data suggest that in addition to anti-self natural Treg cells, a population of Treg cells, activated in the peripheral lymphoid tissues by pathogens may play a major role in controlling excessive inflammatory responses to infection. Natural and pathogen-induced Treg cells are phenotypically and functionally similar in that they express CD25, GITR, and the transcriptional regulatory protein Foxp3, and are immunosuppressive for antigen-activated T cells, suggesting that natural and peripheral activated Treg cells may be of the same lineage. However, we have recently demonstrated in the FIV AIDS lentiviruses infection that pathogen-induced Treg cells differ from natural Treg cells in that they express TGF-2 on their surface (mTGF-2+) and that mTGF-2 mediates Treg suppressor function and may also control peripheral Treg homeostasis. The experiments proposed herein will further explore these observations to test the hypothesis that Treg cells in the peripheral immune tissues are a normal component of the immune regulatory process to infectious agents and that AIDS lentiviruses over-ride the normal controls on Treg activation and function, resulting in chronic immunosuppressive activity and abnormal Treg homeostasis. CD4+CD25+ Treg cells will be assessed phenotypically (mTGF-2+, Foxp3+) and functionally (inhibition of ConA-induced proliferation and IL2 by CD4+ Th cells and IFN-3 by CD8+ T cells) in cats acutely infected with the NCSU1 isolate of FIV. FIV gag ELISA and RT-PCR assays will be performed to determine if Treg activation is associated with virus infection. The role of mTGF-2 in Treg suppressor function will be determined by the use of TGF-2 and TGF-2RII neutralizing antibodies. To investigate the role of mTGF-2 in maintaining peripheral Treg homeostasis in FIV+ cats by recruitment from the CD4+ Th pool, we will incubate mTGF-2+ Treg cells with CD4+ Th cells and analyze the target cells for expression of CD25, Foxp3, mTGF-2 and for suppressor function. To confirm the mTGF-2+ Treg cells induced conversion of Th cells to Treg cells, we will utilize TGF-2 and TGF-2RII neutralizing antibodies to block the conversion process. These studies will address two important immunological issues: 1) how do CD4+CD25+ Treg cells mediate suppression and phenotypic conversion of CD4+ and/or CD8+ Th populations to maintain their homeostasis; and 2) how do AIDS lentivirus infections over-ride the normal controls over Treg cell activation and function, resulting in chronic Treg-induced immunosuppression, as well as abnormal Treg homeostasis. PUBLIC HEALTH RELEVANCE: T regulatory (Treg) cells play a pivotal role in maintaining the balance between protective immune responses and immunopathology associated with primary infections. We believe that AIDS lentivirus infections such as HIV over-ride the normal controls over Treg cell activation and function, resulting in chronic Treg cell activation and global immunosuppression and AIDS. Understanding how HIV co-opts this normal immune regulatory mechanism, which will be addressed in this proposal, will aid in the development of better therapeutic modalities.